An image forming device includes a developing device provided with a developing roller that carries the developer, and a developer cartridge assembled to the developing device in a horizontal direction. The developer cartridge includes an accommodating portion that accommodates therein the developer and juxtaposed with the developing device in the horizontal direction, and an agitator rotatably disposed in the accommodating portion. The accommodating portion has a wall portion adjacent to the developing device, and a single opening is formed in the wall portion at a widthwise center portion thereof (axially intermediate portion of the developing roller).
The agitator is slidingly moved relative to the wall portion so that the agitator is moved past the opening upward. The agitator lifts the toner deposited on a bottom of the accommodating portion, and pushes the toner into the developing device through the opening. Thus, the toner is supplied from the accommodating portion to the developing device.